This invention relates to optical parametric oscillators (OPOs). Optical parametric oscillators use non-linear crystals in order to tune an output over a range of frequencies. The non-linear crystals are placed within a resonator and driven by an intense pump radiation to generate tunable monochromatic light. OPOs use a three-wave process in which the high-frequency pump .omega..sub.p is decomposed into a signal wave .omega..sub.s and an idler wave .omega..sub.i. The relationship of the pump, signal, and idler is governed by the conservation of energy and momentum. Energy conservation requires that the sum of the generated energies, and therefore frequencies, equal that of the pump. .omega..sub.p =.omega..sub.s +.omega..sub.i. Momentum conservation is shown by the phase-making relationship k.sub.p =k.sub.x +k.sub.i. The values k.sub.p, k.sub.s, and k.sub.i are the momentum vectors for the pump, signal, and idler, respectively, and are related by the wavelength .lambda. of each wave by the relation k=2.pi.n/.lambda., where n is the index of refraction.
In uniaxial birefringent crystals such as .beta.-BaB.sub.2 O.sub.4 (BBO), the index can either be ordinary or extraordinary (n.sub.o or n.sub.e). If ordinary, the polarization vector of the light beam within the crystal is orthogonal to the optic axis of the crystal. In this plane, there is no angular relationship to the index. If extraordinary, the polarization vector of the light beam is in the plane of the optic axis, and therefore there is an angular relationship for the index. Momentum matching is achieved by rotating the crystal in the extraordinary plane, thereby varying the index and its associated k vector, of one of the light waves.
In many applications, it is desired that the output of the optical parametric oscillator have a narrow linewidth. Applications in spectroscopy and photochemistry may require linewidths of less than 0.1 cm.sup.-1. Additionally, it is important that the optical parametric oscillator be efficient. This is especially true since some non-linear crystals such as BBO are typically pumped close to their damage threshold.